The present invention relates to systems for receiving broad-band signals and, in particular, to an adaptive filter for suppressing narrow-band interference in such systems.
Modern communications and radar systems often use a broad-band signal format for transmission of information or for the radar function. By phase coding or frequency hopping the transmitted signal it is possible to spread the transmitted energy over a bandwith much larger than required by the modulation content of the signal. Since the effectiveness of the communications link is determined by the total amount of energy in the transmitted signal, it is possible to use a broad-band signal, wherein the total energy is spread over a large frequency band, to limit the amount of energy at any particular frequency and thereby obtain a communications signal which is largely indistinguishable from background noise.
Such broad-band signals are also useful to eliminate mutual interference between signals in a communications system, such as the one described in Grossman's U.S. Pat. No. 3,714,573 entitled "Spread-Spectrum Position Monitoring System," which is assigned to the same assignee as the present application.
In a radar system it is desirable to use a large amount of energy for target detection. By use of a spread-spectrum transmitted signal the total energy may be spread in time as well as in frequency, thereby eliminating the need for a high peak power pulse which may cause breakdown in the radar transmitting equipment. Upon reception the spread-spectrum signal may be compressed in time to form an output pulse which would be equivalent to a radar pulse of higher peak power than actually used in the transmitter. The most common form of spread-spectrum signal used in radar systems probably is the frequency modulated "chirp" pulse. In this type of system a signal is transmitted which varies in frequency from the beginning of the pulse to the end of the pulse. Upon reception this frequency variation is used in a pulse compression filter to achieve a high energy pulse of relatively short duration.
In both radar and communications systems phase modulation may be used to achieve a spread-spectrum signal. In a phase-modulated, spread-spectrum system the transmitted signal is rapidly switched in phase to produce a signal which has a wide frequency band. Upon reception the signal is sampled by a "matched filter" which decodes the transmitted signal and developes a short output pulse in response to the correct phase coded signal. The matched filter provides no output for an incorrectly coded or uncoded signal.
In any of the above-described systems, or similar systems which utilize a broad-band signal, narrow-band signals, such as CW or narrow-band FM can interfere with proper system operation. The high signal level of such narrow-band signals, compared to the noise-like broad-band, desired signal, can cause the narrow-band signal to activate an automatic gain control circuit in the receiver, thereby suppressing the desired broad-band signal.